1. Field of the Invention
This invention relates to a light-emitting device, an illumination apparatus, and a display apparatus. More specifically, the invention relates to a light-emitting device which uses electroluminescence of an organic material, and an illumination apparatus and a display apparatus using the light-emitting device.
2. Description of the Related Art
Light-emitting devices (hereinafter referred to as organic EL devices) which use electroluminescence of an organic material have attracted attention as a light-emitting device capable of emitting high-luminance light with low-voltage direct-current driving and have been actively researched and developed. The organic EL device has a structure in which an organic layer having a light-emitting layer that generally has a thickness of about several tens to several hundreds of nm is interposed between a reflective electrode and a translucent electrode. In such an organic EL device, light emitted from the light-emitting layer is extracted to the outside after undergoing interference in the device structure. In the related art, several attempts have been made to improve emission efficiency of the organic EL device using such interference.
JP-A-2002-289358 discloses a technique in which a distance from an emission position to a reflective layer is set so as to allow light having an emission wavelength to resonate using interference of light emitted from a light-emitting layer towards a translucent electrode and light emitted towards a reflective electrode, thus enhancing emission efficiency.
JP-A-2000-243573 defines a distance from an emission position to a reflective electrode and the distance from the emission position to an interface between a translucent electrode and a substrate by taking reflection of light at the interface between the translucent electrode and the substrate into consideration.
WO01/039554 discloses a technique in which the thickness of a layer between a translucent electrode and a reflective electrode is set so as to allow light having a desired wavelength to resonate using interference of light occurring when light undergoes multiple reflections between the translucent electrode and the reflective electrode, thus enhancing emission efficiency.
Japanese Patent No. 3508741 discloses a method of controlling an attenuation balance of the three colors red (R), green (G), and blue (B) by controlling the thickness of an organic layer as a method of improving the viewing angle characteristics of a white chromaticity point in a display apparatus having a light-emitting device in which emission efficiency is enhanced using a cavity structure.
The techniques mentioned above are directed to an organic EL device which uses interference of emitted light in order to enhance emission efficiency. In such an organic EL device, when the bandwidth of an interference filter for extracted light h narrows, the wavelength of the light h shifts largely when the emission surface is viewed from an oblique direction, and the emission intensity decreases. Thus, the viewing-angle dependency of emission characteristics increases.
In contrast, JP-A-2006-244713 discloses a technique in which the phase of light emission by a reflective layer of an organic EL device having a narrow single-color spectrum and the interference by a single reflective layer provided on the light emitting side are set to be in an opposite phase to the central wavelength, thus suppressing a variation of hue in accordance with a viewing angle.
In an organic light-emitting device having white light-emitting layers stacked sequentially, since the interference as above occurs in the device, in order to effectively extract white emission having a wide wavelength component, it is preferable for the emission position to be positioned close to the reflective layer particularly at a distance of 80 nm or less. When the emission position is separated from the reflective layer and the distance increases, it is difficult to obtain white emission having a wide spectrum due to the interference.
JP-A-2004-79421 discloses a technique in which a distance from the emission position to a reflective layer and a distance from the emission position to the interface between a translucent electrode and an outer layer are defined so as to obtain a high-efficiency light-emitting device having excellent white chromaticity.
JP-A-2006-244712 discloses a technique which enables a good white chromaticity point to be obtained by adopting the interference of the opposite phase as disclosed in JP-A-2006-244713. However, since it is difficult to achieve phase cancellation over a wide wavelength range, similarly to JP-A-2006-244713, there is no discussion regarding suppression of a variation in hue in accordance with the viewing angle as compared to a single color.
JP-A-2006-173550, JP-T-2008-511100, and JP-T-2008-518400, for example, disclose a technique in which a plurality of light-emitting layers is stacked with an intermediate layer disposed therebetween in order to enhance emission efficiency and improve the light emission lifecycle, thus forming an organic layer so as to have a stacked structure (a so-called tandem structure) having light-emitting layers connected in series. In this type of organic layer, an arbitrary number of light-emitting layers can be stacked. In this case, particularly, by stacking a blue light-emitting layer generating blue light, a green light-emitting layer generating green light, and a red light-emitting layer generating red light, it is possible to generate white light as a combined light of the blue, green, and red light.
However, when the tandem structure as above is formed, it is difficult to make all the distances from the respective emission positions to the reflective layer to be 80 nm or less. Moreover, since the viewing-angle dependency of luminance and hue is very high, the intensity distribution properties of an illumination apparatus or the display properties of a display apparatus are greatly decreased.